Treatment of resin surfaces prior to non-electrolytic plating



United States Patent 3,547,785 TREATMENT OF RESIN SURFACES PRIOR TO NON-ELECTROLYTIC PLA'I'lNG Toramitsu Sakuma, Osaka-fin, Japan, assignor to Sumitomo Naugatuck and Company, Ltd., a corporation of Japan No Drawing. Filed Jan. 5, 1968, Ser. No. 695,843 Int. Cl. C23b 5/62, 5/64 US. Cl. 204--30 ABSTRACT OF THE DISCLOSURE The surface of a shaped article made of thermoplastic resin (e.g., an ABS molding) is prepared for non-electrolytic plating with metals by immersing the article in an oxidizing solution containing chromic acid and sulfuric acid while simultaneously carrying out electrolytic reduction in the solution. The electrolytic reduction is accomplished for example by placing a lead anode and a stainless steel cathode in the oxidizing solution, and passing a current through the solution. The adhesion of a subsequent plate (e.g., copper, nickel) applied by non-electrolytic methods is improved, as compared to the adhesion obtained when an otherwise similar pre-treatment is used without the simultaneous electrolytic reduction. The electrolytic reduction enables the oxidizing bath to retain its effectiveness even after repeated use. The non-electrolytic plating may be followed by the usual electroplating.

BACKGROUND OF THE INVENTION Field of the invention The invention relates to non-electrolytic plating of plastics, and particularly to the pre-treatment of a plastic surface which is to be plated subsequently by non-electrolytic methods. The invention also relates to pre-treated and plated plastics so obtained.

Description of the prior art The non-electrolytic plating of thermoplastic resins, particularly of ABS resin, is being practiced extensively in order to compensate for the shortcomings of the resins such as, for example, insufficient surface hardness and lack of conductivity. In non-electrolytic plating of a thermoplastic molding, it is a general practice to treat the molding, prior to plating, with a mixed solution of chromic acid and sulfuric acid so that the surface of the molding is made hydrophilic (see, for example, Kobunshi (High Polymer, Japan), vol. 14, p. 740 (1965)).

It has been the general practice to use a chromic acidsulfuric acid solution repeatedly for a considerable period of time to treat a large number of the plastic articles to be plated. However, such repeated use has a drawback in that the oxidizing power of the solution decreases gradually with repeated use, even if the chromic acid and sulfuric acid are replenished to restore the desired concentration of the acids. This results in the lowering of the adhesion of a subsequently applied non-electrolytic plating. Thus, when a specimen of an ABS molding pretreated by oxidation with a freshly mixed solution of chromic acid and sulfuric acid at a minimum rate of 1 liter of solution per square decimeters of the molding is compared with another specimen similarly treated with a solution which has already been used once, the specimen pre-treated with the used solution displays approximately lower adhesion of a non-electrolytic plating although the concentration of hexavalent chromium ions has decreased only by about 2% by weight.

8- Claims "ice SUMMARY OF THE INVENTION The invention is based on the discovery that excellent adhesion of a non-electrolytic plating to the surface of a shaped organic plastic article can be obtained if, during the pre-treatment of the plastic article with an oxidizing solution comprising chromic acid and sulfuric acid, there is simultaneously carried out an electrolytic reduction of chromium ions in the solution. The invention is further based on the discovery that the solution can be used repeatedly for such a pre-treatment. and will continue to provide good adhesion of a subsequently applied nonelectrolytic plating, provided that the pre-treatment is accompanied by simultaneous electrolytic reduction in the solution. The non-electrolytic plating may be followed by the usual electroplating by conventional methods. The metals which may be plated include copper, nickel, chromium, tin, aluminum, silver and gold, in various combinations of successive layers if desired.

The present invention provides a method which permits repeated use of the mixed solution of chromic acid and sulfuric acid for surface oxidation treatment used in the non-electrolytic plating of a thermoplastic molding.

Further, the present invention provides a method by which the surface oxidizing force of a used mixed solution of chromic acid and sulfuric acid is stronger than that of a fresh chromic acid-sulfuric acid mixed solution.

In addition, the present invention provides a method by which plated adhesion is improved at such areas of an injection molding as the rem otest points from the gate position, the middle or the concave parts of complex surfaces where the adhesion is normally poor and thus variations in plated adhesion of a plated molding are minimized almost to zero.

Therefore, the present invention is a non-electrolytic plating method for thermoplastic resin moldings, which is characterized in that the surface of a thermoplastic resin molding is pretreated in a mixed oxidizing solution of mainly chromic acid and sulfuric acid while the electrolytic reduction of chromium ions in solution is carried out simultaneously and thereafter chemical plating is applied thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The oxidizing solution used for the pretreatment under the invention comprises chromic acid and sulfuric acid, as in conventional practice. A solution of sulfuric acid with chromic anhydride or an alkaline salt of chromic acid dissolved therein is generally used. A higher concentration of sulfuric acid in the mixed solution is preferable in that the oxidizing power of the chromium ions is much increased. However, when the concentration is too high, it sometimes tends to deteriorate the thermoplastic resin properties. Thus, it is preferable to use sulfuric acid at a concentration less than about 60 percent by weight. On the other hand, if the concentration is too low, for ex ample, less than about 38 percent by weight, the mixed solution does not have suflicient oxidizing power. The sulfuric acid may be substituted, up to about 10 percent by weight, with other strong acids such as nitric acid, phosphoric acid or acetic acid, with good results. However, too much substitution with such acids should be avoided since it shortens the life of the solution.

The solution must be supersaturated with chromic acid or a chromate at room temperature as well as at the temperature at which the pretreatment is rendered. For example, when the concentration of sulfuric acid in the solution is 50 percent by weight, addition of 7 percent .by weight, or more, as chromic anhydride is preferable. As a compound to provide hexavalent chromium ions in the oxidizing solution, chromic anhydride alone may be used, but it is preferable to let it co-exist in the solution with an alkaline salt of chromic acid such as, for example, sodium dichromate or potassium dichromate. Together with the trivalent chromium ions produced in the mixed oxidizing solution, the sodium or potassium ions form the chromate salt, insoluble in the solution, and after precipitation the salt is removed from the system so that the generation of nascent oxygen is activated, thus accelerating the oxidation of the surface of the molding.

The materials of the electrodes used for the electrolytic reduction under the present invention may be any suitable conventional materials, and may comprise, for example: lead for the anode, and an iron or chromium material for the cathode such as, for example, stainless steel, chromium-aluminum-steel, Nichrome, high silicon steel, austenite cast iron, etc. The current density at the cathode is preferably kept below two amperes per square decimeter, and preferably the electrodes have an area ratio such that the cathode has an area twice (as a maximum) that of the anode. Hydrogen is generated near the cathode. Therefore, the molding is kept as far as possible from the electrodes while the solution is mechanically stirred or has air blown in for carrying on the electrolytic reduction more effectively.

The method of the invention is applicable to conven tional organic plastics, including a wide range of thermoplastic resins such as, for example, ABS, acrylonitrilestyrene copolymer, polyethylene, polystyrene, polypropylene, polyvinyl chloride, polyvinyl acetate, and like vinyl resins. Particularly good results are obtained when it is applied to resins having unsaturated radicals such as ABS resin, etc., notably copolymers of such dienes as butadiene, isoprene, 2,3-dimethylbutadiene-l,3, pentadiene or similar conjugated diolefins (containing, for example, 40% of combined diene, the remainder of the copolymer being derived from any suitable copolymerizable monoethylenically unsaturated monomer or monomers such as styrene, acrylonitrile, methyl methacrylate, alpha-methyl styrene, etc.).

The ABS resin may be either of the graft copolymer type (e.g. graft copolymer of acrylonitrile and styrene on polybutadiene or butadiene-styrene copolymer), or it may be of the polyblend type (e.g. blend of styrene-acrylonitrile resin with butadiene-acrylonitrile rubber), or it may be a combination of the two (e.g. blend of the said graft copolymer with separately prepared styrene-acrylonitrile resin). Some or all of the acrylonitrile and/or styrene may be replaced by equivalent materials such as methacrylonitrile, acrylic esters (e.g. methyl methacrylate, ethyl acrylate), alpha-methyl styrene, dichlorostyrene, etc.

The present invention will now be described by the examples set forth hereunder.

EXAMPLE 1 An ABS resin made by mixing 55 parts by weight of a graft copolymer (prepared by graft copolymerizing 50 parts of styrene and acrylonitrile [weight proportion, 70/30] on 50 parts of polybutadiene rubber) and 45 parts of a separately prepared resinous copolymer of styrene and acrylonitrile (weight proportion, 70/30) is injection molded into flat test specimens of 2 x 50 x 60 mm. using a 5 ounce screw type single shaft injection molding machine and a single cavity mold. The mold has a standard gate of 1 mm. square on the top left edge of the flat plate. The injection conditions are: resin temperature, 250 C.; mold temperature, 70 C.; injection pressure, about 680 kg./ sq. cm. and back-pressure, 340 kg./ sq. cm., and the cycle may be: injection time, seconds; holding time, 25 seconds; cooling time, seconds, and mold opening, ejection, mold closing, 20 seconds. The treatment of the moldings in the chromic acid-sul furic acid mixed solution may then be carried out by the following four different procedures and the samples are identified accordingly.

(A) A mixed solution of chromic acid and sulfuric 4 acid containing grams of chromic anhydride and 40 grams of sodium dichromate and having a sulfuric acid concentration of percent by weight (i.e., one liter of the solution contains 40 g. of chromic anhydride, 40 g. of sodium dichromate and 50% by weight of sulfuric acid [as 100% P1 1) is prepared and left as such for more than two days and nights. A test specimen having a surface area corresponding to 10 sq. dm. is dipped in the solution and treated at a temperature of 60i5 C. for 15 minutes, during which time the solution is stirred.

(B) A lead anode with a surface area of 0.8 sq. dm. and a stainless steel cathode with a surface area of 1.0 sq. dm. are placed 100 mm. apart in 1 liter of the mixed solution similarly prepared as in A above. Test specimens are dipped and treated similarly to process A, while a direct current, 2 volts and 2 amperes, is passed through the electrolyte solution.

(C) Test specimens are treated under the same conditions with those employed in A above in the solution once used for A.

(D) Test specimens are treated in the used solution for A while electrolytic reduction as in B is simultaneously carried out in the solution.

The surface oxidized test specimens, samples A, B, C and D, may be washed with water. Then, they are dipped for 5 minutes in a dilute aqueous solution of hydrochloric acid, wherein stannous chloride is dissolved (eg. solution containing about 4 g./l. of hydrochloric acid [as 100% HCl] and about 20 g./l. of stannous chloride; the pH is suitably greater than 5.0 and the hydrochloric acid content is suitably less than 25% that of the stannous chloride), and thereafter for 30 seconds in a dilute aqueous solution of hydrochloric acid, wherein palladium chloride is dissolved (e.g. solution containing 0.08 g./l. of hydrochloric acid, [as 100% HCl] and 0.1 g./l. of anhydrous palladium chloride; the pH is suitably greater than 5 .0 and the hydrochloric acid content is suitably less than 25 that of the palladium chloride), and washed with water. Thereafter, they are dipped in a solution of chemical copper for 15 minutes to form a layer of chemical copper thereon with a thickness of about 0.3 micron. The chemical copper solution may be made up as follows:

CuSO 5I-I O 35 NiCl -6H 0 8 'Formaldehyde (37%) 100 Rochelle salt (aqueous) 150 NaOH (to give pH 12) 40 Thiourea (stabilizer) 0.001

The specimens are then dipped in dilute sulfuric acid for a little over 10 seconds, and immediately plated with electrolytic copper to a thickness of about 40 microns in a bath of copper sulfate containing no lustering agent. The copper sulfate bath suitably may have the following composition:

The bath may have a pH of less than 1.4 and a specific gravity of greater than 1.3 at room temperature. The current density may be 5 amp./dm. maintained for 90 minutes. The electrode may be electrolytic copper. The plated moldings are dipped in warm water of about C. for 15 minutes for uniform adhesion. The test specimens are taken out and left at room temperature for more than 48 hours.

The specimens, thus plated, may be subjected to tests to determine resistance against peeling, heating and cooling. The peel test may be carried out in the following manner. Two rectilinear scratches are cut at a spacing of 10 millimeters on the plated surface. The plated layer is pulled at an angle of degrees and a speed of millimeters per minute with an Instron tester, and the force required for peeling is read on the scale of the tester. The heating and cooling test comprises repetition of a cycle of heating at 80 C. for hours, standing at room temperature for 30 minutes, cooling at C. for 7 hour-s and standing at room temperature for minutes, cooling at -20 C. for 7 hours and standing at room temperature for 30 minutes. The number of cycles, during which no swelling or peeling of the plated film is observed on the plated surface, determines the degree of resistance to environmental conditions.

The results of the tests are shown in Table 1.

TABLE 1 Peel strength (g./cm.)

Remotest Heating and area from cooling test gate (cycles to position failure) Near gate position Center part Samples may be prepared exactly as in Example 1, except that an oxidizing mixed solution of the following composition may be used as the chromic acid-sulfuric acid mixed solution:

Grams/liter Sulfuric acid 480 Orthophosphoric acid Chromic anhydride 40 Sodium dichromate 40 The results of the tests conducted on the plated specimens with respect to peel strength and resistance against heating and cooling are shown in Table 2.

TABLE 2 Peel strength (g./cm.)

Remotest Heating and area from cooling test gate (cycles to position failure) Near gate position Center part EXAMPLE 3 Plating may be applied under the same method as Example 1, with exception of the use of a solution of chemical nickel in place of the solution of chemical copper and formation of a film of chemical nickel in a thickness of about 0.2 micron. The solution of chemical nickel may be any suitable conventional preparation, as represented by such commercially available materials as Sumer brand (Nippon Kanizen Company), and may have for example the composition (published by US. Bureau of Standards in 1940 by Dr. Brenner): 1

NiSO -7H O 40 Sodium citrate 24 Sodium hypophosphite 20 Sodium acetate 14 Ammonium chloride 5 The tests may be carried out exactly as in Example 1. The results of the tests are shown in Table 3.

TABLE 3 Peel strength (g./cm.)

Remotest Heating and Having thus described my invention, what I claim and desire to protect by Letters Patent is:

1. In a method of pre-treating the surface of a shaped thermoplastic resin article, and subsequently non-electrolytically plating of a metal onto said surface, wherein during said pre-treatment the article is immersed in an oxidizing solution comprising chromic acid and sulfuric acid to render the surface hydrophilic, the improvement comprising:

effecting electrolytic reduction of chromium ions in said solution during said pre-treatment by passing a direct electric current through the solution between an anode and cathode while the resin article is immersed therein.

2. A method as in claim 1, in which the thermoplastic resin is an unsaturated copolymer of butadiene.

3. A method as in claim 1, in which the thermoplastic resin is ABS.

4. A method as in claim 1 in which the said solution contains from 38% to 60% of sulfuric acid, and is supersaturated with chromic acid.

5. A method as in claim 1 in which up to 10% by weight of the sulfuric acid is substituted by nitric acid,

phosphoric acid or acetic acid.

6. A method as in claim 1 in which said electrolytic reduction is carried out by passing a direct current between an anode and cathode in said solution while mechanically stirring the solution or blowing air therethrough.

7. A method as in claim 6 in. which the said anode comprises lead, and the said cathode comprises iron or chromium.

8. A method as in claim 1, in which the pre-treated, non-electrolytically plated surface is thereafter electroplated.

References Cited UNITED STATES PATENTS 3,035,941 5/1962 Cohen et al. 117-47 3,142,581 7/1964 Leland 117-47 3,235,426 2/1966 Bruner 117-47 3,248,271 4/1966 Rielly et al. -1 156-2 3,370,974 2/ 1968 Hepfer 117-47 3,445,350 5/1969 Klinger et al. 204-20 3,423,300 1/1969 J00 et a1. 204-97 OTHER REFERENCES Chemie-Ing.-Techn., vol. 35, No. 5, January 1963, Elektrlytische Regeneration Von Chromsaure, Dr. Mario Kappel, pp. 386-389.

DANIEL E. WYMAN, Primary Examiner P. E. KONOPKA, Assistant Examiner US. Cl. X.R. 

